Change test.sh to output JUnit-XML

[artasparks]

We should change test.sh to output XML -- then Prow will split up the tests into separate sections. I think that's trivial with the current setup, coming from this post.

Basically, just need to use py.test --junitxml results.xml tests.py or similar.

Another option is just to use Bazel for everything, which has this feature in a language-agnostic way.